It is known to heat treat a metallic workpiece in a fluidized bed of fire-resistant refractory particles under an inert gas atmosphere at a treatment temperature of 500.degree. to 650.degree. C. The heated workpiece is contacted in the fluidized bed with a gas mixture composed of nitrogen, ammonia, and a carbon-rich gas to nitrocarburize it.
The temperature of the gas used in this heat treatment determines the oven atmosphere. Such fluidized-bed heat treatment is characterized by good heat transfer so that the treatment time is short. In addition in such a system the gas atmosphere can be modified in an extremely short time by changing its composition, as the large volume of gas that must pass through the bed to fluidize it ensures that the changes per unit of time are large so that any change in the composition of the fluidizing gas is immediately apparent. Thus the inert-gas atmosphere used during the heating and the thermochemical atmosphere during the actual heat treatment can be clearly separated (Gas Heat International 33, pp. 290 to 295 (1984) [Gaswarme International 33 (1984), S. 290 to 295]).
Such a process is typically used for nitrocarburizing, which is a gas nitriding process associated with a carbon uptake. Thus a surface or compound layer forms on the workpiece which can have up to 10% by weight nitrogen and up to 2% by weight carbon and a minimum thickness of 5 micron to 15 micron. The compound layer formed by nitrocarburizing is characterized by an improved wear resistance relative to the base material of the workpiece and an improved corrosion resistance.
U.S. Pat. Nos. 4,512,821 and 4,524,957 describe a process wherein the gas composition remains constant during the nitrocarburization. Nothing is done to the workpiece in the fluidized bed before the nitrocarburizing to pretreat the workpiece. As a result the surface layer formed by the nitrocarburizing is porous to a considerable extent at least at the outer surface and to a lesser extent throughout the cross section of the surface layer. The pores reduce the hardness and the wear resistance of the surface layer formed by nitrocarburizing.